I am searching for a tool or method to access paritions from a file. So
for example I have a simple file or logical volume which contains a
partition table and multiple partitions. So I normally can't mount this
partitions with mount.

In Linux for example I would simply use kpartx to get
dummy/pointer-devices for the partitions in /dev/mapper/ so is there a
tool which do the same on NetBSD to get access to the partitions within
the file or logical volume?